I have neither. I use an old Garmin Forerunner 201. On my Tarmac that is the only instrument - no speedo.
i like the GPS because I can down load the ride to the computer for me records, I also like it for club rides with que sheets, since the distance is exact I know exactly where I am on the que sheet.

So to give you my thoughts - I would stay with the GPS but get the features you need. The two features I wish I had are cadence and a barometric altimeter. I don't want a physically big package - which is what I like about the 201.